Two genes involved in the determination of segment identity in Drosophila were studied: female (1) sterile homeotic (fsh), and trithorax (trx). The fsh gene is required maternally for proper differentiation of body pattern, whereas trx acts predominantly in the zygote. Mutations in fsh and trx interact strongly, leading to high levels of homeotic transformations of a bithorax type. To ask whether this interaction is specific, a large fraction (about 20%) of the entire genome was tested for dosage-dependent maternal interactions with trx or zygotic interactions with fsh. In addition to several regions that gave effects of low penetrance, one chromosomal region was found that showed strong interactions. This region, defined by a deletion, contains the ash-1 gene that is known from previous work to interact with fsh and trx, and the kohtalo gene, suspected of such interactions. The paucity of regions in the genome affecting the expression of fsh and trx suggests that the synergism between mutations in these two genes is based on highly specific interactions. A molecular analysis of the trx gene has been continued with the expression of segments of the very large open reading frame (3759 amino acids) as fusion proteins in bacteria and yeast. In this way the entire trx protein has been expressed. As predicted from the sequence it was shown that certain regions of the protein are capable of binding zinc in vitro. This finding is compatible with but does not establish the hypothesis that the trx protein is a member of the zinc finger family of transcription factors. Several trx fusion proteins have been used for the production of antisera that will be employed in the characterization of the trx protein.